FIG. 1 is a block diagram showing a configuration of an ATM-PDS (Asynchronous Transfer Mode Passive Double Star) system as a conventional data transmission system. In FIG. 1, the reference numeral 101 designates a central office unit comprising multiple transmitting and receiving sections, although only one transmitting and receiving section 114 is shown for simplicity. The reference numeral 102 designates a star coupler as an optical distributor that is connected to the central office unit 101 via an optical fiber 103; 104a-104c each designate an optical fiber connected to one of split output terminals of the star coupler 102; and 105a-105c each designate a subscriber unit connected to one of the optical fibers 104a-104c. Since the split number of a single star coupler is 32 at present, the total of 32 subscriber units can be connected to each star coupler by connecting them to the split output terminals via the optical fibers 104a-104c . . . . 
The central office unit 101 comprises a transmitting laser diode (LD) 112 for outputting a video signal generated by a video signal generator 111 in the form of an optical signal; a wavelength division multiplexer/demultiplexer (WDM) 113 supplied with the output of the transmitting laser diode (LD) 112 and the output of the transmitting and receiving section 114; an electric signal multiplexer/demultiplexer 115; and a processing section 116. The transmitting and receiving section 114 includes a wavelength division multiplexer/demultiplexer (WDM) 121; a receiving photodiode (PD) 123 for converting an optical signal supplied from the wavelength division multiplexer/demultiplexer (WDM) 121 into an electric signal; a transmitting laser diode (LD) 122 for converting an electric signal to an optical signal; and a signal processor 124. The processing section 116 includes a signal processor 117, a transmitting laser diode (LD) 118 and a receiving photodiode (PD) 119.
The subscriber unit 105a comprises a wavelength division multiplexer/demultiplexer (WDM) 131a connected to the fiber 104a; a receiving photodiode (PD) 132a for receiving a wavelength band of a video signal separated by the wavelength division multiplexer/demultiplexer (WDM) 131a and for outputting it as an electric signal; a video receiver 133a supplied with the electric signal; and a transmitting and receiving section 134a supplied with signals other than the video signal separated by the wavelength division multiplexer/demultiplexer (WDM) 131a. The transmitting and receiving section 134a includes a wavelength division multiplexer/demultiplexer (WDM) 141a; a receiving photodiode (PD) 142a for converting an optical signal fed from the wavelength division multiplexer/demultiplexer (WDM) 141a into an electric signal; a transmitting laser diode (LD) 143a for converting an electric signal into an optical signal; an electric signal multiplexer/demultiplexer 144a; an A/D (Analog/Digital) converter 145a to which a telephone 147a is connected; and an A/D (Analog/Digital) converter 146a to which a facsimile machine 148a is connected. A personal computer 149a is directly connected to the electric signal multiplexer/demultiplexer 144a. The subscriber unit 105b connected to the optical fiber 104b has a similar configuration. When no video receiver is required as in the subscriber unit 105b, a terminator 135b is connected in place of the receiving photodiode (PD).
Next, the operation will be described.
In the central office unit 101, the video signal generator 111 supplies its video signal to the transmitting laser diode (LD) 112. The transmitting laser diode (LD) 112 supplies it to the wavelength division multiplexer/demultiplexer (WDM) 113 in the form of the optical signal. The wavelength division multiplexer/demultiplexer (WDM) 113 multiplexes the optical signal with the optical signal from the transmitting and receiving section 114, and supplies it to the star coupler 102 via the optical fiber 103. The star coupler 102 splits the signal and supplies the split signals to the subscriber units 105a, 105b and the like.
In the subscriber unit 105a, the wavelength division multiplexer/demultiplexer (WDM) 131a demultiplexes the input signal into the video signal and the other signals, and supplies the video signal to the video receiver 133a via the receiving photodiode (PD) 132a. On the other hand, the signals other than the video signal are supplied to the receiving photodiode (PD) 142a via the wavelength division multiplexer/demultiplexer (WDM) 141a in the transmitting and receiving section 134a, to be converted into the electric signal. Then, the electric signal multiplexer/demultiplexer 144a demultiplexes the electric signal into respective signals so that the telephone signal is supplied to the telephone set 147a via the A/D converter 145a, and the facsimile signal is supplied to the facsimile machine 148a via the A/D converter 146a. As for the computer signal, the electric signal multiplexer/demultiplexer 144a supplies it directly to the personal computer 149a. 
On the other hand, as for the signals from the devices connected to the subscriber unit 105a such as the signal from the telephone set 147a, for example, the A/D converter 145a converts it to the digital signal, and supplies it to the transmitting laser diode (LD) 143a via the electric signal multiplexer/demultiplexer 144a. The transmitting laser diode (LD) 143a converts it to the optical signal, and supplies it to the star coupler 102 via the wavelength division multiplexer/demultiplexers (WDMs) 141a and 131a. The star coupler 102 sends it to the central office unit 101 via the optical fiber 103, where it is supplied to the receiving photodiode (PD) 123 via the wavelength division multiplexer/demultiplexers (WDMs) 113 and 121, to be converted into the electric signal and output. The output signal passes through the signal processor 124 and the electric signal multiplexer/demultiplexer 115, and is supplied to the processing section 116, where it passes through the signal processor 117, and is converted to the optical signal by the transmitting laser diode (LD) 118, again, to be transmitted to another station.
In the foregoing conventional data transmission system, it is considered preferable to divide the wavelength range 1480-1580 nm, which is assigned to the downlink signals from the central office unit to the subscriber units, into two regions of 1480-1530 nm and 1530-1580 nm, and to assign the longer wavelength region 1530-1580 nm to video signal deliverance. In this case, the subscriber unit requires the wavelength division multiplexer/demultiplexer (WDM) that demultiplexes the wavelength region 1480-1580 nm assigned to the downlink signal into the wavelength region 1530-1580 nm for the video signal and to the wavelength range 1480-1530 nm assigned to the signals other than the video signal.
As a typical conventionally used wavelength division multiplexer/demultiplexer (WDM), a spatial optical filter is known. FIG. 2 shows a spatial optical filter. It comprises a glass substrate 151, on a side of which a reflecting layer 152 is formed that reflects a particular wavelength signal. It further comprises, at both sides of the glass substrate 151, condenser lenses 153 and 154 which are coupled with the optical fibers 155 and 156, respectively, and a condenser lens 157 coupled with an optical fiber 158 in such a manner that the reflected light off the reflecting layer 152 is launched into the optical fiber 158 through the condenser lenses 157.
As described above, the spatial optical filter has a complicated configuration. In particular, it is difficult to align the optical axes of the optical fiber and of the condenser lenses, increasing the total cost. Since the expensive spatial optical filter is installed in the subscriber unit to separate the video signal and the signals other than the video signal, the subscriber unit is costly. This offers a problem in that a subscriber who does not want to receive the video service must purchase the expensive subscriber unit.
The present invention is implemented to solve the foregoing problem of the conventional system. Therefore, an object of the present invention is to provide an inexpensive subscriber unit for a subscriber who does not want to receive the video service.